1. Field of the Invention
The present invention relates to an internal combustion engine and more specifically to an air-fuel control system for an internal combustion engine which is arranged to be operated on gasoline and/or mixtures of gasoline and alcohol (e.g. alcohol mixed with methanol and/or ethanol).
2. Description of the Prior Art
In the event that it is desired to operate an internal combustion engine on either gasoline or a mixture of gasoline and one or more alcohols, in order to control the amount of noxious emissions it is insufficient to only modify the ignition timing and it is essential to determine the amount of alcohol in the fuel. That is to say, as the amount of alcohol in the fuel can fluctuate markedly, without knowing the amount which is contained in the fuel being supplied, the amount of noxious emissions which are emitted from the engine cannot assuredly be attenuated to the desired level.
JP-A No. 56-98540 discloses an example of an arrangement which has, been proposed in order to control the supply of fuel to the engine in accordance with the amount of alcohol contained therein. In this arrangement, a capacitance type alcohol sensor is disposed in the conduit through which fuel containing alcohol is supplied. The output of this sensor is indicative of the amount of alcohol which is contained in the fuel and is used to simultaneously correct both the amount of fuel which is supplied to the engine and the ignition timing.
However, this arrangement has suffered from the drawback that this system has been designed to correct the fuel supply amount in response to the output of the alcohol sensor and has not been arranged to cope with the situation wherein the alcohol sensor malfunctions.
Accordingly, in the case where the sensor does in fact malfunction, the control of the amount of fuel which is supplied to the engine deteriorates and induces a corresponding deterioration in the air-fuel ratio control which can lead to stoppage of the engine.